Alternate polymer extrusion system and method with accumulator

ABSTRACT

An extrusion system and method uses an accumulator to control and use excess plastic melt. The accumulator is situated between an extruder and a gear pump. The accumulator includes an accumulator housing to store excess plastic melt from the extruder and an accumulator spring connected to an accumulator piston to subsequently send the excess plastic melt to the gear pump. In another embodiment, the accumulator includes an accumulator housing to store excess plastic melt from the extruder and a hydraulics system to send the excess plastic melt to the gear pump. The accumulator can be a first in first out (FIFO) device having a melt passage through the accumulator piston. The system and method are used particularly advantageously in Alternate Polymer extrusion installation in which gear pumps regularly increase and decrease their speed but in which the melt-supplying extruder cannot accommodate such rapid speed changes.

RELATED APPLICATIONS

This application claims priority from U.S. provisional patentapplication Ser. No. 60/586,672, filed Jul. 9, 2004, entitled “AlternatePolymer Extrusion System and Method With Accumulator,” which is herebyincorporated herein.

FIELD OF THE INVENTION

This invention relates to control systems and methods for plasticsextruders, and more particularly to such systems and methods with anaccumulator module.

BACKGROUND OF THE INVENTION

In an Alternate Polymer™ co-extrusion system, two or more extrudersdeliver two or more plastic melts to gear pumps connected at the outputof the extruders. The outputs of the gear pumps are connected to aco-extrusion die. The gear pumps are controlled to alternately delivermore or less of their corresponding melt, requiring the gear pumps tovary their operating speed. Pressure sensors between each gear pump andits associated extruder detect any abrupt pressure differential acrossthe gear pump. Such a pressure differential indicates that the gearpump's speed has changed, and that the extruder should change its speedto ensure proper feed of the plastic melt from the extruder to the gearpump. Alternate Polymer systems of this kind are described in the U.S.Pat. Nos. 5,725,814 and 5,695,789, issued Mar. 10, 1990 and Dec. 9,1997, respectively in the name of Holton E. Harris and assigned to theassignee of the present invention. These two patents are incorporatedherein by reference.

If an extruder can respond to a speed change in its associated gearpump, the plastic melt will flow without significant problems. However,an extruder in operation has certain inertia which resists a speedchange in the extruder. Thus, large extruders have a significant amountof inertia, which prevents them from quickly adjusting to speed changesin the gear pump. Generally, in practice, an extruder can adjust to thespeed changes in the gear pump where is the extruder has a relativelysmall diameter such as up to 1 inch or 1.5 inch diameter. However, inextrusions systems where the extruder has a larger diameter, such as,for example, in extrusion systems that produce corrugated tubing, theextruder may not be able to adjust effectively to speed changes in thegear pump. This results in the problem of having too much or too littleplastic melt supplied to a gear pump from the extruder, which preventsthe effective operation of the extrusion system.

BRIEF SUMMARY

In accordance with the invention an accumulator is located along thepath of melt flow between the extruder output and the gear pump input.

An accumulator may be generally described as a container for storingexcess plastic melt when such melt is not needed and for delivering themelt when needed. An exemplary accumulator may be spring-loaded. In sucha spring-loaded accumulator, the unneeded plastic from the extrusionsystem enters the accumulator and pushes against a side of the containersupported by a spring. Once the excess plastic is needed, the springpushes the plastic back into the extrusion system. Therefore, theaccumulator will take up excess plastic when there is excess and deliverit back when the excess is needed. The accumulator solves the problem ofhigh extruder inertia causing a delivery of too little or too much meltto the gear pump when the gear pump changes speed. To allow the extruderand the gear pump to operate together, the accumulator accepts plasticmelt from the extruder when the gear pump makes a significant reductionin speed and the accumulator then releases plastic melt to the gear pumpwhen the gear pump increases speed.

Ordinary spring-loaded accumulators are LIFO (last in first out)devices. In other words, the last of the plastic melt that is stored inthe accumulator is the first to be expelled when additional melt isrequired. Therefore, it is often likely that the oldest melt (which willbe the last out) is left in the accumulator for a number of operationsof the extrusion system. Since unused plastic is subject to degradation,the oldest melt may degrade while waiting to be returned to the meltpath.

In one preferred embodiment of the invention the accumulator has aplastic melt delivery path that delivers plastic melt into anaccumulator cylinder, through or past an accumulator piston and past acheck valve, to a reservoir location where the plastic melt isalternately accumulated and expelled. The plastic melt delivery paththrough (or past) the piston is in series in the plastic melt deliverypath from the extruder to the gear pump, so the first plastic melt to bedelivered into the reservoir of the accumulator is the first tosupplement the supply to the gear pump when the gear pump increasesspeed. Thus, this embodiment of the accumulator is a FIFO (first infirst out) accumulator. It solves the above-noted problem of the LIFOaccumulator.

In an exemplary embodiment of the invention, two or more extrudersdeliver two or more plastic melts to gear pumps connected at theiroutputs. The gear pumps are connected to a co-extrusion die in anAlternate Polymer extrusion system. To alter the plastic content in theextrudate along its length, the gear pumps are controlled to alternatelydeliver more or less of their corresponding melt, alternately speedingup and slowing. It should be noted that some feed forward control of theextruder may be used as is known so that the pressure change that comesfrom abruptly altering gear pump speed is anticipated and slowing orspeeding up of the extruder is not solely in response to the sensedpressure differential. However, by placing an accumulator in each linebetween the extruder and the gear pump each extruder can respond moreslowly than its associated gear pump. Each accumulator accepts plasticmelt when the pressure at the input to the gear pump is sufficient todrive a biased piston back into a cylinder under pressure from theplastic melt. Likewise, when the gear pump rapidly comes back up tospeed but the extruder lags behind, the pressure drop in the plasticmelt at the input to the gear pump allows the accumulator piston to pushforward previously stored melt as the extruder comes up to speed.

The above and further objects and advantages of the invention will bebetter understood by reference to the following detailed description ofone or more preferred, exemplary embodiments taken in consideration withthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified schematic illustration, in block diagram form, ofan alternate polymer extrusion system with an accumulator piston in afirst position in accordance with a first embodiment of the invention.

FIG. 2 is a simplified schematic illustration, in block diagram form, ofthe alternate polymer extrusion system like that of FIG. 1 with anaccumulator piston in a second position in accordance with the firstembodiment of the invention.

FIG. 3 is a simplified schematic illustration, in block diagram form, ofthe alternate polymer extrusion system of FIG. 3 with an accumulator ina first position in accordance with a second embodiment of theinvention.

FIG. 4 is a simplified schematic illustration, in block diagram form, ofthe alternate polymer extrusion system of FIG. 3 with an accumulator ina second position in accordance with the second embodiment of theinvention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A first embodiment of an extrusion system having an accumulator is shownin FIG. 1. The extrusion system 10 includes two or more extrusionmodules. The first extrusion module 20 is diagrammatically shown ingreater detail and additional extrusion modules are depicted as a blockn. Each of the additional extrusions modules n may be the same orsimilar to the first extrusion module 20. The first extrusion module 20and each additional extrusion module 20 is connected with a co-extrusiondie 30.

As shown, the first extrusion module 20 has an extruder 40, a gear pumpor melt pump 50 (referred to hereafter solely as “gear pump”) and a LIFO(“last in first out”) accumulator 100 disposed along a melt path 70. Acontroller 80 is connected to and controls the extruder 40 and gear pump50. An example of a suitable controller 80, for the purposes describedherein, is the CP-873 or CP-874 Digipanel controller commerciallyavailable from Harrel, Incorporated of East Norwalk, Conn. as describedat the website page http://www.harrel.com/upgrtemp/index.html,incorporated herein by reference.

The extruder 40 heats plastic into a hot viscous plastic mass (called“plastic melt” or “melt”) and moves the plastic downstream along themelt delivery path 70 to the left in FIG. 1. The extruder 40 may be, forexample, any commercially available extrude such as the one of BOBCATseries of extruders available from Harrel, Incorporated. These also aredescribed at the incorporated by reference website pagehttp://www.harrel.com/bobcat/index.html. The controller 80 controls thespeed and temperature of the extruder 40.

The gear pump 50 functions to force a precisely metered quantity of themelt through the die to form an extrudate. The gear pump 50 may increaseits delivery of plastic melt by increasing speed, or reduce its deliveryof plastic melt by reducing speed. Preferably, the gear pump 50 iscapable of quick acceleration and deceleration when driven by itsassociated gear pump drive 60 (a suitable servo-motor, for example). Thecontroller 80 controls the gear pump drive 60. The gear pump 50 may alsobe commercially available gear pump much as one of those offered byHarrel, Incorporated and described at the Harrel website pagehttp://www.harrel.com/.

The accumulator 100 of the first embodiment is located along the meltdelivery path 70 between the extruder 40 and the gear pump 50. Theaccumulator 100 includes an accumulator housing 110 defining anaccumulator storing location or reservoir 102. The melt delivery path 70continues from an input end 106 of the accumulator 100 through theaccumulator melt storing location 102 and out of the accumulator housing110 at an output end 108 of the accumulator 100. A check valve 112 islocated at the input end 106. The accumulator 100 also includes anaccumulator piston 114 slidable in the accumulator housing 110. Theaccumulator piston 114 is biased into its first position shown in FIG. 1by a spring 116.

In operation, as illustrated in FIG. 1, with the gear pump 50 and theextruder 40 both operating, plastic melt passes from the extruder 40along the melt delivery path 70 which extends through the accumulator100, past the gear pump 50 and to the co-extrusion die 30.

To completely alternate the melt plastic in the extrudate emerging fromthe die 30, one or more of the additional extrusion modules n ramps up,the gear pump 50 comes to a halt and the extruder 40 slows. The meltmoves through the accumulator 100. When the gear pump 50 slows or comesto a halt, the increase in pressure above the accumulator piston 114forces the accumulator piston 114 down within the accumulator housing110 to the position shown in FIG. 2.

When the gear pump 50 is restarted to deliver plastic to theco-extrusion die 30, the accumulator piston 114, biased by the spring116, pushes excess melt located in the melt storing location 102 upwardtoward the gear pump 50 as a result of pressure above the accumulatorpiston 114 being relieved by the start, or by the increase in speed, ofthe gear pump 50. The check valve 112 stays seated in the input end 106,so upward movement of the accumulator piston 114 forces the accumulatedplastic above the accumulator piston 114 to the gear pump 50, exhaustingthat stored supply of melt before fresh melt from the extruder 40 passesthrough the accumulator piston 114 to the gear pump 50, ending with theelements of the first extrusion module 20 again in the positionillustrated in FIG. 1.

The foregoing explanation of operation of the Alternate Polymer systemof FIGS. 1 and 2 is directed to operation of the system to changeentirely from one plastic to another, by completely stopping andstarting each of the gear pumps. It should be noted that a variation inrelative content or ratio of the two plastics along the length of theextrudate less than an entire change-over from one to the other may bedesired. This invention will benefit any such arrangement as well.

FIGS. 3 and 4 show a preferred embodiment using a FIFO (“first in firstout”) accumulator 200. The accumulator 200 has an accumulator piston 202slidable in an accumulator housing 204. A shaft or tie rod 206 connectsthe accumulator piston 202 to a hydraulic system 208 that includes ahydraulic drive piston 210 in a hydraulic cylinder 212. A hydraulicpower unit 214 applies pressure to oil in the hydraulic system 208. Ahydraulic accumulator 216 maintains a relatively constant pressure tothe oil of the hydraulic system 208 under the influence of a nitrogensupply 218 as is known.

Leading from the extruder 40 to the gear pump 50, the melt delivery path270 of this embodiment includes a passage 220 through the accumulatorpiston 202 as illustrated in FIG. 3. A check valve 224 is located at anoutput end 226 of the passage 220.

In operation, as illustrated in FIG. 3, with the gear pump 50 and theextruder 40 both operating, plastic melt passes from the extruder 40through the accumulator piston 202 of the accumulator 200, through thegear pump 50 to the co-extrusion die 30.

To alternate the melt plastic, one or more additional extrusion modulesn ramp up, the melt pump 50 comes to a halt, and the extruder 40 slows.The plastic melt moves through the accumulator piston 202. The increasein pressure above the accumulator piston 202, caused by the stoppinggear pump, forces the accumulator piston 202 down within the accumulatorhousing 204 to the position shown in FIG. 4. The space above the piston202 thus accumulates the continued melt flow from the extruder 40 asthat extruder slows.

The rod 206 connects the accumulator piston 202 to the hydraulic piston210 in the cylinder 212. Thus, the accumulator piston 202 is retractedpushing oil into the hydraulic accumulator 216. The hydraulic power unit214, in a preferred and exemplary embodiment, maintains about 800 PSIpressure in the hydraulic system 208. The hydraulic accumulator 216 isbacked up with the nitrogen supply 218 that maintains the pressure onthe oil in the hydraulic system 208.

When the gear pump 50 is restarted to deliver melt to the co-extrusiondie 30, the oil in the hydraulic accumulator 216 pushes the hydraulicpiston 210 upward in the hydraulic cylinder 212 as pressure above theaccumulator piston 202 is relieved by the gear pump 50. The check valve224 stays seated in the opening out of the passage 220 throughaccumulator piston 202, so upward movement of the accumulator piston 202first forces the accumulated plastic melt above the accumulator piston202 to the gear pump 50, exhausting that stored supply of theaccumulated plastic melt before fresh melt from the extruder 40 passesthrough the accumulator piston 202 to the gear pump 50, ending with theelements of the first system again in the positions illustrated in FIG.3.

Again, although the example that has been given contemplates completestopping of the gear pumps to change the extrudate from one plastic toanother, the system can be operated such that the pumps do not stopcompletely and the relative content of two or more plastics varieslengthwise along the extrudate.

In extrusion systems using accumulators in accordance with thisinvention, because pressure in the accumulator can become extremelyhigh, it may be necessary to take steps to prevent the welding togetherof the accumulator piston and housing, even in the case where these aremade of differing metals. In one instance this was done by increasingthe clearance between the piston and housing interior surface more thanone ordinarily would choose and by surface hardening the piston andhousing interior surfaces by Borofuse processing. The Borofuse processis a hardening process available from Materials Development Corporationof Medford, Mass.

While one or more specific preferred embodiments have been describedherein, those skilled in the art will readily recognize modifications,variations and equivalents that do not depart from the spirit and scopeof the subject invention, as herein claimed.

1. An extrusion system comprising: an extruder for moving plastic internally toward an end thereof; an accumulator, connected downstream to the extruder, for receiving plastic from the extruder; a gear pump, connected downstream to the accumulator, for receiving plastic from the accumulator; and a die, connected downstream to the gear pump, for receiving plastic from the gear pump and for thereafter forming the plastic into an extrudate, wherein the accumulator collects plastic from the extruder in response to a first operating condition of the gear pump, and wherein the accumulator provides plastic to the gear pump in response to a second operating condition of the gear pump.
 2. The extrusion system of claim 1, wherein the first operating condition of the gear pump is a change in speed of operation of the gear pump.
 3. The extrusion system of claim 1, wherein the second operating condition of the gear pump is a change in speed of operation of the gear pump.
 4. The extrusion system of claim 1, wherein the accumulator further comprises: a housing for receiving the plastic; and a piston, biased by a spring, for moving the plastic within the accumulator to the gear pump.
 5. The extrusion system of claim 4, wherein the extrusion system further comprises a check valve positioned between the extruder and the accumulator, wherein the check valve prevents plastic in the accumulator from flowing to the extruder while the plastic in the accumulator is being sent to the gear pump.
 6. The extrusion system of claim 1, wherein the accumulator further comprises: a housing for receiving the plastic; and a piston, connected to a hydraulics system, for moving the plastic within the accumulator to the gear pump.
 7. The extrusion system of claim 6, wherein the extrusion system further comprises a check valve positioned between the extruder and the gear pump, wherein the check valve prevents plastic in the accumulator from flowing to the extruder while the plastic in the accumulator is being sent to the gear pump.
 8. The extrusion system of claim 1, wherein the accumulator is a first in last out accumulator.
 9. The extrusion system of claim 1, wherein the accumulator is a first in first out accumulator.
 10. The extrusion system of claim 1, further including at least one further extruder, accumulator and gear pump, the die being a co-extrusion die connected with each of the gear pumps, whereby alternately increasing and decreasing the speeds of the gear pumps alters the relative content of plastic delivered via the co-extrusion die to the extrudate by the gear pumps.
 11. The extrusion system of claim 1, the accumulator comprising: a housing; a piston with the housing; and the path of flow of plastic from the extruder into the accumulator includes a passage through the piston to a plastic accumulation locator in the accumulator.
 12. The extrusion system of claim 11, further comprising a check valve located in the passage to prevent plastic flowing from the accumulation location back to the extruder.
 13. A method of extrusion comprising: extruding plastic from an extruder; forcing the plastic downstream from the extruder to a gear pump through an accumulator; collecting plastic from the extruder into the accumulator in response to a first operating condition of the gear pump; and providing plastic from the accumulator to the gear pump in response to a second operating condition of the gear pump.
 14. The method of extrusion of claim 13, wherein the first operating condition of the gear pump is a change in speed of operation of the gear pump.
 15. The method of extrusion of claim 13, wherein the second operating condition of the gear pump is a change in speed of operation of the gear pump.
 16. The method of extrusion of claim 13, wherein the step of collecting plastic comprises receiving the plastic in a housing, and the step of providing plastic comprises using a spring-biased piston to push the plastic towards the gear pump.
 17. The method of extrusion of claim 16, further comprising controlling the flow of plastic to the gear pump by positioning a check valve between the extruder and the accumulator, wherein the check valve prevents plastic in the accumulator from flowing to the extruder while the plastic in the accumulator is being sent to the gear pump.
 18. The method of extrusion of claim 13, wherein the step of collecting plastic comprises receiving the plastic in a housing, and the step of providing plastic comprises using a hydraulics-activated piston to push the plastic towards the gear pump.
 19. The method of extrusion of claim 18, further comprising controlling the flow of plastic to the gear pump by positioning a check valve between the extruder and the accumulator, wherein the check valve prevents plastic in the accumulator from flowing to the extruder while the plastic in the accumulator is being sent to the gear pump.
 20. The method of extrusion of claim 13, wherein the last plastic collected during the collecting step is the first plastic provided during the providing step.
 21. The method of extrusion of claim 13, wherein the first plastic collected during the collecting step is the first plastic provided during the providing step.
 22. The method of extrusion of claim 13, further comprising: extruding plastic from a further extruder; forcing the plastic downstream from the further extruder to a further gear pump; through a further accumulator, providing a co-extrusion die connected to an output of each gear pump; and alternately increasing and decreasing the speeds of the gear pumps to alter the relative content of delivered via the co-extrusion die to the extrudate by the gear pumps.
 23. The method of extrusion of claim 13, wherein the step of collecting plastic from the extruder into the accumulator comprises delivering plastic to an accumulation location in the accumulator through a passage in a piston within a housing of the accumulator.
 24. The method of extrusion of claim 23, further comprising preventing plastic flow from the accumulation location back to the extruder by a check valve in the passage in the piston.
 25. An extrusion system comprising: means for extruding plastic; means for providing the extruded plastic to a gear pump; means for accumulating excess plastic from the extruder which was not sent to the gear pump; and means for subsequently providing the excess plastic to the gear pump.
 26. The extrusion system of claim 19, further comprising: means for controlling the means for extruding and the gear pump to vary the speed thereof.
 27. The extrusion system of claim 26, further comprising further means for extruding, plastic, means for providing the extruded plastic to a gear pump and means for accumulating excess plastic, each gear pump being connected at its output to a co-extrusion die, and means for controlling comprising means for alternately varying the speeds of the gear pumps and extruders to vary the relative content in an extrudate of plastics delivered from the gear pumps; whereby the means for accumulating accumulate excess plastic each time the associated gear pump shows more quickly than the associated extruder and provides the excess plastic to the gear pump each time the associated gear pump accelerates more quickly than the associated extruder. 